Both the notebook computers and the portable electronic apparatus use the cold cathode fluorescent lamp as a backlight because this lamp has the best illumination efficiency. Therefore, the cold cathode fluorescent lamp has quickly been adopted for use as the backlight in PDAs, notebook computers and portable electronic apparatus. The quality requirement of the converter for the cold cathode fluorescent lamp is also increased.
A high voltage DC/AC converter is required to drive the cold cathode fluorescent lamp because this lamp uses a high AC operation voltage. However, with the increasing size of the LCD panel, the panel requires multiple lamps to provide the necessary illumination. Therefore, an effective converter is required to drive multiple cold cathode fluorescent lamps. The driving technique requires careful treatment.
FIG. 1 shows a schematic drawing of a circuit structure for an DC/AC converter used to drive two cold cathode fluorescent lamps in accordance with the prior art. DC power 100 provides DC power to the full bridge circuit 102. DC power 100 is connected to a primary winding 104 of a transformer through the full bridge circuit 102. The secondary winding 106 of a transformer is coupled to two cold cathode fluorescent lamps 112 and 114 through two high voltage capacitors 108 and 110, respectively. A half-bridge circuit, a push-pull circuit or a Royer circuit can be used to replace the full bridge circuit 102. However, this circuit structure does not ensure that each cold cathode fluorescent lamp connected with the circuit structure is ignited successfully. The characteristics of the cold cathode fluorescent lamp is negative resistance and the voltage needed to ignite the lamp is different under various conditions such as aging of the lamp, temperature of the lamp and parasitic coupling between and lamp and the metal chassis. For example, one of the two cold cathode fluorescent lamps connected in this circuit structure is severely aged, the circuit cannot ignite the lamp due to the voltage at the transformer decreases once the other lamp has ignited. This, in turn, decreases the life-span of the cold cathode fluorescent lamps.
FIG. 2 shows a schematic drawing of another circuit structure schematic drawing for a DC/AC converter that used to drive two cold cathode fluorescent lamps in accordance with the prior art. DC power 100 provides DC power to the full bridge circuit 102. DC power 100 is connected to a primary winding 104 of a transformer through the fill bridge circuit 102. The secondary winding 106 of a transformer is coupled to two cold cathode fluorescent lamps 112 and 114 through an inductor 116 and two high voltage capacitors 108 and 10, respectively. A half-bridge, a push-pull or a Royer circuit can be used to replace the full bridge circuit 102. However, this circuit structure uses an inductor 116 between the secondary winding 106 and two high voltage capacitors 108 and 110, which may cause this circuit structure to be affected easily by an operation frequency associated with a DC/AC power converter. The variation of operating frequency may cause different AC currents to flow through the two cold cathode fluorescent lamps 112 and 114, respectively. In addition, this circuit structure is also sensitive to load variations. Therefore, if this circuit structure is used to drive multiple cold cathode fluorescent lamps, it is difficult to balance the current flowing through each lamp. Moreover, circuit design is difficult and complicated.
FIG. 3 shows a schematic drawing of a circuit structure of a plurality of transformers that are used to drive a plurality of cold cathode fluorescent lamps in accordance with the prior art. It is used to solve the problems described in the two circuit structures shown in FIG. 1 and FIG. 2. DC power 100 provides DC power to the full bridge circuit 102. DC power 100 is connected to two primary windings 104a and 104b through the full bridge 102. The secondary windings 106a and 106b are coupled to two cold cathode fluorescent lamps 112 and 114 through two high voltage capacitors 122 and 124, respectively. A half-bridge circuit, a push-pull circuit or a Royer circuit can be used to replace the full bridge circuit 102. Although this circuit structure increases the reliability and stability, structural formation of this kind of DC/AC converter for driving a cold cathode fluorescent lamp is expensive. Furthermore, a DC/AC converter with this circuit structure is bulky.